A wireless power system employs a pair of inductive coils forming a loosely-coupled transformer to transfer power wirelessly. Power is transferred through the system more efficiently by using impedance-matching networks, which cancel the reactive impedance of the inductive coils. In the ideal case with a resistive load, the matching networks can be tuned perfectly to cancel all reactive impedance, creating a perfectly matched circuit. In practice, the load power varies according to the demands of the device or devices being serviced, resulting in a varying load impedance. The coupling factor between the transmitter and receiver coils changes with the relative position of the charger and the charging device.
FIG. 1 illustrates a wireless power system that includes a wireless power transmitter 2 and a wireless power receiver 3. The wireless power transmitter 2 receives a fixed voltage from a DC adapter. The fixed adapter voltage is scaled by a DC/DC converter 4 and applied to an inverter 6. The inverter, in conjunction with transmitter matching network 8, generates an AC current in transmit coil 10. The AC current in the transmit coil 10 generates an oscillating magnetic field in accordance with Ampere's law. The oscillating magnetic field induces an AC voltage into the tuned receiver coil 12 of wireless power receiver 3 in accordance with Faraday's law. The AC voltage induced in the receiver coil 12 is applied to a rectifier 16 that generates an unregulated DC voltage. The unregulated DC voltage is regulated using a DC/DC converter 18, which is filtered and provided to a load, such as a battery charger of an electronic device.